Because of the primary commercial importance of abalone among marine animals as classified above, the invention is described with specific relation to abalone culture, although it is not intended to be so limited.
Many species of abalone and other forms of edible shellfish in the natural state are rapidly being depleted by predators, pollution, and commercial and sport harvesting, to the point that there is no longer an adequate source of these shellfish to satisfy world demand. The price of edible abalone meat has escalated seven-fold in the last ten years.
The normal life cycle of an abalone starts with a spawning process involving the dissemination of eggs by a female into a seawater environment and subsequent fertilization by male sperm. Spawning has been successfully introduced in commercial mariculture operations in Japan and the United States.
The fertilized abalone eggs, which are about 150 microns in diameter, undergo several stages of development in the first 24 to 36 hours, and then hatch to become free-swimming larvae. These larvae hatch without a protective shell, but within approximately six hours an initial shell is developed. The ability of the larvae to swim is provided by a velum, which includes many hairlike cilia that beat rapidly to propel the animal through the water. During this larval stage, it is believed that the primary source of nutrition is obtained from the egg yolk still contained within the body of the larva. Over an approximately four-day period, under controlled conditions, and this time period may be longer in nature, most of the larvae undergo physiological and morphological development, including the development of a foot, after which they begin seeking a suitable surface upon which to settle and metamorphose. This foot provides a means to crawl upon hard surfaces, and when a suitable surface is found, they attach to the surface, lose their velum and then begin to metamorphose from a larval to a juvenile abalone form. This process of metamorphosis, which involves a number of complex physiological and morphological changes occupies several days. The larvae, which are about 150 microns in greatest dimension at the time of hatching, grow to an approximate size of 250 microns in the four-day period while they are free-swimming.
When the swimming larvae reach the stage of development prior to settling which, as noted above, occurs in approximately four days, they begin to search for a suitable substrate upon which to settle. Upon sensing a suitable surface, a larva will settle and change from a swimming animal to a surface crawling animal, followed by the initiation of a number of other changes that result in the formation of a juvenile organism and the commencement of active feeding.
This application is directed to procedures for optimizing the settling and metamorphosis of abalone larvae and the survival and rapid growth of the young settled animals in mariculture conditions.
In nature, when the larvae are competent to metamorphose, they select suitable surfaces upon which to settle by temporarily ceasing to manipulate their swimming mechanism, thereby allowing gravity to gently pull their bodies to the ocean floor. When a larva settles upon a horizontal surface on the sea floor, it extends its newly developed foot and attempts to attach itself to the solid substrate upon which it has landed. Should it find this surface to be be biologically, chemically or physically unsuitable, it again manipulates its swimming mechanism and swims upward into the water column to repeat the process. When a larva finds a surface with suitable characteristics, it settles and rejects its velum and becomes a crawling animal. The searching process for a substrate in good culture conditions usually begins on the fourth day after hatching. It has been observed, however, that the searching process can last for as long as 30 days if suitable substrate conditions are not available.
The 60-day period immediately following settlement involves a critical period in the abalone's life. As soon as a larva settles and changes from a swimming to a crawling snail-like gastropod, it actively moves about on its settlement surface and begins feeding. For the first three to seven days, the young post-larval animal ingests bacteria, yeast, fungi, protozoa, and possibly other microorganisms generally less than five microns in size. At this state, the abalone's mouth is a small, ill-defined opening incapable of ingesting larger size particles.
During the first five to ten days of growth, the animal's mouth rapidly enlarges to a size capable of handling five to ten micron size phytoplankton that are ingested as the abalone crawls along the surface rasping food particles growing on this surface. During the next 60 days the young abalone continues to grow rapidly with the development of a mouth structure, which, at the end of this period, can ingest particles 200 microns or more in size. Many of the larvae, both in nature and cultured under mariculture conditions, fail to survive the first 60 days. Our mariculture process provides a substantial improvement over nature in survival rates during the critical 60-day period when we believe most young abalone perish, as well as improvements over other abalone culturing methods. Such prior art methods are described in the following publications:
1. "The Abalone Science and Its Propagation in Japan", Takashi Ino, (original title in Japanese, "Awabi To Sono Zoyoshoku"), Vol. 11 in Series on the Propagation of the Marine Products, 1966, published by Nippon Suisan Shigen Hogo Kyokai.
2. "Abalone", Masaaki Inoue, Vol. I in Marine Product Culture Data Book, 1976, published by Suisan Shuppan (Marine Printing).
3. "Laboratory Observations on the Early Growth of the Abalone, Haliotis Sorenseni, and the Effect of Temperature on Larval Development and Settling Success", David L. Leighton, published in Fishery Bulletin, Vol. 70, No. 2, 1972.